1. Field of the Invention
The present invention relates generally to data distribution and more particularly to passenger aircraft data distribution.
2. Description of the Related Art
It is increasingly common to have telephone handsets provided at each seat complex in modern passenger aircraft. These handsets are the passenger's entry into two-way voice communication, which is connected to ground-based telephone networks over various "bearer" radio links that include satellite services.
In a typical aircraft telecommunications network, a central telecommunications unit interfaces between the bearer links and a cabin distribution system which includes electronics distribution modules positioned at each seat complex (usually a 2 or 3 seat group). One or more telephone handsets extend from each distribution module for use by the passengers in that seat group.
In an aircraft environment, a plurality of voice signals are preferably distributed in a digital format. Accordingly, standards have been established for digital aircraft telephone signal formats. A commonly used standard directed to cabin distribution systems is known as CEPT El which provides for 2.048 Mbps transmission (CEPT is an acronym for Committee for European Postal Telecommunications). This standard establishes a frame of 32 time slots or "channels". Each time slot carries 8 bit digital data with thirty time slots dedicated to audio data channels and two slots reserved for handshaking, synchronization and channel assignment. The frame repeats at an 8 KHz rate, which is sufficient for voice communication. Digital data in this format is hereinafter referred to as E1 digital data. At each seat distribution module, a coder-decoder (CODEC) typically performs two-way conversion between the E1 digital data and the analog signal of the passenger handsets.
The CEPT E1 standard originally envisioned transmission over two twisted pair cables; one twisted pair cable for differential transmission in each direction. However, existing and planned audio and video entertainment systems also utilize the cable sets that extend from the central telecommunications unit. Because of weight, size and cost constraints, the number of cable sets is limited and there is increasing pressure, e.g., from aircraft manufacturers, to allocate only a single set of twisted pair cables for passenger telephone distribution.
Although this would free a cable set for other communication use, allocation of only one cable set for two-way telephone data distribution raises several technical risks. Because of their structure, twisted pair cables inherently reduce signal coupling from external sources. Since both wires have nearly equal exposure to externally generated radiation, they couple substantially equal signals from this radiation. If the twisted pair cable is differentially received, these coupled radiation signals tend to be canceled. Conversely, the twisted pair of wires usually carry a differential signal, i.e., the signal on one wire has the opposite polarity of the signal on the other wire. Thus, radiation out of the twisted pair cable also tends to cancel and its interference with external systems is consequently reduced.
Telephone distribution in a passenger aircraft requires long transmission paths, e.g., &gt;60 meters. Absent the coupling protection of a differentially-driven, twisted pair cable, the E1 data signal may become so distorted that data recovery and synchronization are jeopardized. At the same time, radiated emissions may interfere with other passenger service communications.
One method of maintaining two-way telephone communication with a single set of twisted pair cables is to time share the use of that set. Without an increase in transmission data rate, time sharing reduces the number of available telephone channels, which is an undesirable reduction in passenger service. If the transmission data rate is doubled, the number of channels will not be reduced but the higher data rate increases various technical risks, e.g., excessive emitted radiation and data recovery faults.